1. Field of the Invention
The present invention is broadly concerned with magnetic element temperature sensors, detectors for use with such sensors, closed-loop heating systems making use of the sensors and detectors to wirelessly determine the temperature of an object and to control the object's temperature, and corresponding methods. More particularly, the invention is concerned with temperature sensors made up of at least one magnetically susceptible sensor element, preferably formed of amorphous or nanocrystalline metal, and having a re-magnetization response under the influence of an applied alternating magnetic field which is different below and above at least one set point temperature, such as the Curie temperature of the sensor element. These temperature sensors can be used with correlated detectors for temperature sensing, and as a part of closed-loop heating systems. The temperature sensors may be incorporated into adhesive backed stickers that can be quickly and easily adhered to any number or type of objects, for example servingware, so that the servingware, and food place thereon, can be heated by a closed-loop feedback heating system.
2. Description of the Prior Art
A variety of applications for temperature sensors that can be read wirelessly exist in the marketplace. These applications vary from sensing and reporting the internal temperature of livestock to being part of a closed-loop temperature feedback system that allows a magnetic induction heater to precisely control the temperature of insulated food delivery boxes. Many of these applications are disclosed in U.S. Pat. Nos. 5,954,984, 6,232,585, 6,320,169, 6,953,919, and 6,208,253.
Many of those applications are currently being served by Radio Frequency Identification (RFID) temperature sensing systems. These RFID temperature sensing systems include an RFID reader/detector and its associated RFID “tag,” whereby the tag has some type of temperature sensor as part of its circuitry.
These prior REID systems tend to be relatively expensive owing to the cost of the tags, and are unable to operate continuously in excess of 125° C. Moreover, they lack the ability to transmit information in the vicinity of metal or other conducting materials, particularly when the RFD tag is embedded within the conducting material.
Magnetic element markers (or “tags”) are commonly used as part of an electronic article surveillance (EAS) systems or other authentication systems. These markers or tags are passive, typically small, less expensive than RFID tags, can operate at high temperatures, and in some forms, can transmit their information wirelessly to a detector even when embedded within a conductor.
For example, EAS markers or tags made of soft magnetic amorphous alloy ribbons are disclosed in U.S. Pat. No. 4,484,184. These ribbons have a composition consisting essentially of the formula Ma Nb Oc Xd Ye Zf, where M is at least one of iron and cobalt, N is nickel, O is at least one of chromium and molybdenum, X is at least one of boron and phosphorous, Y is silicon, Z is carbon, “a”-“f” are in atom percent, a ranges from about 35-85, b ranges from about 0-45, c ranges from about 0-7, d ranges from about 5-22, e ranges from about 0-15 and f ranges from about 0-2, and the sum of d+e+f ranges from about 15-25. The marker ribbons are capable of producing field perturbations at frequencies which are harmonics of the frequency of an incident alternating magnetic field produced by a field transmitter. A detecting means is arranged to detect magnetic field perturbations at selected tones of the harmonics produced in the vicinity of the interrogation zone by the presence of the marker therewithin. Generation of harmonics by the marker is caused by nonlinear magnetization response of the marker to an incident magnetic field.
There is a need in the art for wireless temperature sensing systems using small, less expensive temperature sensing elements, that can operate continuously at temperatures in excess of 125° C., and that have the ability to transmit information even in the vicinity of metal or other conducting materials. Furthermore, it would be advantageous if such improved temperature sensing elements were able to carry predetermined data relating to the sensor itself or to the object to be temperature sensed, e.g., the identity of the object, object characteristics, or heating instructions. Finally, advantages would be realized if the sensing elements could be used as a part of a closed-loop feedback heating system able to control the output of a heating device and thus control the temperature of an object.
In connection with another aspect of the present invention, restaurants and other food-serving establishments commonly use various devices to keep servingware (dinner plates, platters, bowls, pans, chafing dishes etc.) and the food thereon or therein warm after the food is placed upon the servingware. For example, plates of food prepared in restaurants must be kept warm while other plates of food destined for the same customer are still being prepared. Similarly, warming trays are frequently used to keep serving platters and bowls and the food thereon warm.
The most common devices for keeping servingware and food warm are heat lamps which use radiation from various light sources; food wells, or steam tables, which use conduction from condensing steam generated via energy from either a petroleum-fueled flame below, or an electric element submerged in, a water bath below the servingware; and microwave ovens. Unfortunately, these heating devices are inefficient and have no convenient means to precisely control the temperature of the servingware or food and thus frequently overheat or underheat the servingware and food.
For example, with heat lamp systems, the lamps are on continuously, even when there is no servingware below the lamps, thus wasting energy and unnecessarily heating surrounding areas. Furthermore, these systems have no temperature feedback from the food to the heat lamp and therefore continue to heat the food after it has been warmed above its proper temperature, resulting in overheating and drying of the food. To reduce the likelihood of overheating, heat lamps with reduced power ratings are sometimes used, but undersized heat lamps often do not generate enough energy to fully heat the servingware to a temperature high enough to keep the food thereon warm enough. Finally, the rim of servingware placed under heat lamps often gets hotter than desired because the heat lamps direct light upon the rim as well as the food on the servingware. This necessitates the use of gloves or pot holders when handling the servingware and wastes energy used to unnecessarily heat the rim.
Similarly, steam table systems are energy inefficient because they have no closed loop temperature feedback. Thus, to ensure safe food temperatures, these systems are typically operated at their highest temperatures, wasting energy and causing the food heated by the systems to become overheated and dried-out.
Microwave ovens also typically do not use temperature feedback information to allow closed loop temperature control of servingware placed therein. Some microwaves have temperature probes that can be inserted into food to provide temperature information so as to create a closed-loop temperature control system. However, such wired probes are not convenient, especially for high volume operations such as restaurants.
Thus, there is a need in the art for improved devices, systems, and methods for maintaining the temperature of food items after they are placed on or in servingware.